It is known to use a basic oscillator and one or more frequency synthesizers to generate stable local oscillator frequencies for the modulator of the transmitter and for the demodulator of the receiver. For the reception a mixing frequency is formed, with which the received RF signal is shifted to the intermediate frequency, and for the transmission a mixing frequency is formed, with which the intermediate frequency is raised to the transmission frequency. In systems where the difference of the intermediate frequencies used for transmission and reception equals the duplex interval it is possible to use the same mixing frequency for the transmitter and the receiver, and thus in such systems only one synthesizer is required to generate the mixing frequencies. The mixing frequency here relates basically to the signal supplied to the local oscillator port of the mixer, of the modulator, or of the demodulator. In system where the difference of the intermediate frequencies used for transmission and reception is not equal to the duplex interval at least two synthesizers are required to generate the mixing frequencies for the transmitter and the receiver.
Different systems use different frequency bands and so for systems operating on these different frequency bands there has to be synthesizers supplying different frequencies. Thus it is necessary to construct and manufacture different circuits to generate the required frequencies for mobile phones of different systems. For example, manufacturers of mobile phones intended for the GSM (Groups Special Mobile) and PCN (Personal Communications Network) mobile phone systems have to manufacture different RF parts for the GSM and PCN telephones, although the systems are generally similar digital cellular radio systems, except that the GSM operates on the 900 MHZ frequency band and the PCN on the 1800 MHz frequency band. Design and manufacture of different circuits is expensive, because the RF parts of radio telephones comprise several hundred components. Further the size of the batch to be produced will affect the circuit price in that in a smaller batch the unit price of the circuits will be higher than in a large batch.